The invention pertains to helicopters, and more particularly to the improved performance of the circulation control slot portion of an anti yaw system, which replaces the conventional tail rotor system.
Tail rotors have been the almost exclusive means for providing yaw control in helicopters as well as overcoming the biasing torque produced by the main rotor. However, tail rotors contribute to over 15% of all helicopter accidents, largely through tail rotor strikes. The tail rotor also dominates the helicopter's acoustical signature and contributes substantially to the pilot's work load.
A substitute anti-torque and yaw control system employing a combination of a circulation-control tail boom and a jet thruster in place of the tail rotor is taught in U.S. Pat. No. 4,200,252, Logan et al, and owned by the same assignee as the instant case. In this system low pressure air, provided by a variable pitch axial flow fan mounted in the helicopter fuselage, is ejected from thin horizontal slots in the right side of the tail boom. The jets produced by this air flow follow the contour of the tail boom and induce the main rotor wake to do the same. This action produces lift on the tail boom, like any other airfoil, in the direction required to counteract the torque produced by the main rotor. The force produced by the circulation control tail boom is supplemented by the force produced by the jet thruster, which also produces all the yaw maneuvering forces. The proportion of the force produced by each of these two elements of the invention depends upon the mode of flight of the helicopter. In high-speed translational flight, the main rotor wake trails the tail boom and as a result, the tail boom produces essentially no force and all the force is produced by the jet thruster. When the helicopter is in the hover mode, the circulation control on the tail boom provides approximately 60% of the anti-torque force required, while the jet thruster produces the balance. The circulation control tail boom produces a force with relatively low power consumption compared to the jet thruster and power is provided by the engine which also drives the main rotor. Hence it is important for the circulation control tail boom to be as effective as possible. This is particularly true when the helicopter is flying in a hover mode or experiencing low translational flight velocities avoiding the increased power required by the jet thrusters until the aircraft has passed through translational lift. At this point the main rotor power required is decreasing as the fan power required to drive the thrusters is increasing and the "NOTAR".TM. does not limit the aircraft performance with respect to payload.
"NOTAR" is an acronym for "No Tail Rotor" and is a trademark of McDonnell Douglas describing the anti-torque and yaw control system above, which replaces the conventional tail rotor.
It is an object of this invention to improve the performance of the circulation control tail boom when the helicopter is in the hover mode or traveling at low translational velocities.